Electric piano



I Sept. 20; 1949. F, KERKHQF 2,482,548

l ELEcTRIc NANO Filed April 25, 194e VOLT/96e JM/M y ATTORNEY Patented Sept. 20, 1949 ELECTRIC PIANO Frederik Kerkhof, Eindhoven, Netherlands, assignor, by mesne assignments, to Hartford National Bank and Trust Company, Hartford,

Conn., as trustee Application April 25, 1946, Serial No. 664,745

In the Netherlands July 15, 1942 Section 1, Public Law 690, August 8, 1946 Patent expires July 15, 1962 (Cl. 84-L26) 6 Claims.

The present invention relates to electrical musical instruments and more particularly to an instrument playable by keys and having for each key thereof a system whose output potential controls the intensity of the note produced by striking said key.

It is already known, that the course of the decrease in intensity of the sound produced by a string which vibrates out can be approximately obtained with the aid oi a decreasing electric voltage whose value decreases according to an e-function. This decrease of voltageis obtained when a condenser is discharged through one or more ohmic resistances, in which event the voltage at this condenser determines the intensity and the course of the tone.

With the known circuit-arrangements of this kind which are utilized in electric musical instruments, more particularly in electric pianos, the voltage which determines the sound intensity varies according to a single e-function. This result is obtained by the use of a circuit arrangement with which the charge of the condenser gets an opportunity to ow away during a comparatively short or long period according to the force with which the key is touched, the residual charge being transferred to a circuit which consists of a condenser shunted by an ohmic resistance. This condenser is discharged in this case via the ohniic resistance, owing to which the voltage at the condenser decreases according to an e-function.

Although with the above-mentioned circuit arrangement an approximation of the dying away of a tone produced by a key which has been touched is obtained, it has been found that, more particularly for the comparatively low frequencies, which are consequently produced in the piano by touching comparatively long and thick strings, the tone which dies down after being electrically produced is not a faithful reproduction of the tone produced by touching a string.

According to the invention, use is made of a circuit arrangement similar to that previously described but the condenser to which, after the touch, the charge is transferred has connected in parallel to it such impedances that the voltage decreases according to the sum of at least two e-functions.

With the circuit arrangement according to the invention we obtain a long sounding-out period f so that also in the regio-n of the intermediate and low tones of the piano a satisfactory approximation of the mechanical musical instrument is obtained.

According to one particular embodiment of the 55 invention, the condenser to which the charge is transferred after the key has been touched, has connected in parallel to it the series-connection of an ohmic resistance and a capacity which previously has been completely discharged and which is shunted by an ohmic resistance.

With this circuit arrangement it is achieved that the discharged capacity functions at rst as a short-circuit for the resistance by which this condenser is shunted so that the initial discharge of the condenser takes place through a smaller resistance than the further discharge when the condenser shunted by the resistance is charged and acts as an insulator.

Further features of the invention will be explained in detail with reference to the accompanying drawing wherein Fig. 1 represents the circuit arrangement which permits to obtain the desired course of the sound intensity.

Fig. 2 shows the variation oi the voltage at the condenser which determines the instantaneous sound intensity of a tone.

In Fig. 1, S1 denotes a switch Which is actuated by the hammer connected with the key K of the musical instrument, each key of the whole keyboard having such a switch S1. This switch has three contacts, viz, It, Il and I2, which, when the key is depressed, successively enter into contact with the arm of the switch. In the position of rest of the key the arm of the switch stands upon the contact l0, the condenser C1 being no-w connected, through the intermediary of this contact, to a source of direct-current voltage. In the position of rest of the key the condenser C1 is consequently always charged. When one of the keys is touched, the arm of the switch S1 concerned moves from the contact I0 along the contact H to the contact l2. The duration of this movement depends upon the force with which the key is depressed. During this period oi time the arm passes along the contact il and the condenser C1 is partly discharged through a resistance R1. The value oi the residual charge depends upon the time during which the arm touches the contact ll and therefore upon the force with which the key is touched. When the switch arrives upon the Contact l2, a condenser Cz is connected in parallel with C1 so that now the residual charge of the condenser C1 is divided over the condensers C1 and C2. Starting from a determined direct-current voltage at the Contact l, the final charge of the condenser Cz only depends, after the key has been depressed, upon the speed at Whic-h the arm of the switch S1 moves and therefore upon the force with which the key is touched. When the key is completely depressed the arm of the switch S1 gets free from the contact I2 so that the charge remained on the condenser C1 can no longer be transferred to the circuit present beyond the said contact; it is only the charge imparted to the condenser C2 which is decisive for the voltage variation inthis circuit. When the key is released, the arm of the switch S1 remains out of contact with the; contacts II and I2 and it is only when the keyis completely released that it arrives againl upon` the contact I0. The condenser C is charged again and the key may be touched anew.

On the same shaft of the switch S1 ismounted' a switch Sz whose movement is consequently also initiated when the key is touched. The. switch S2 is constructed, however, in such manner that the Contact IS is only closed during the downward movement of the key as long as the arm of the switch S1 has not yet reached the contact I2`- In the further movement of this switch the contact I6 is opened again. AlsoY during the return movement of the switches S1 and S2 theV contact Iiis not closed. When the key is being depressed, but before the tone proper is struck, the switch S2 consequently brings about the short-circuiting of the condenser C3 so that at the beginning of each tone the said condenser is completely discharged. If the time interval between two successive touches of the same key is suflicientlyA long, .this condenser is completely discharged through the resistance R3, it is true, but inthe case of touches which rapidly succeed one another, such as trills, there would exist the possibility, in the absence of the switch S2, of the condenser C3 being not completely discharged at the beginning of each tone. The variation of the voltage in the .circuit might therefore be different from that which is required for obtaining the natural course of the sounding-cut.

The circuit arrangement represented in Fig. 1 is -composed as follows: In parallel with the condenser C2 is connected the series-connection ofv a resistance R2 and a. condenser C3 shunted. by a resistance R3. Before each tone setsin the condenser Cs is always completely discharged byr the above-described switch S2. Furthermore, this condenser C2 has connected in parallelwith it two further series-connections of aA resistancey and a condenser, viz. Re, Ci and'Rq, C5 respectively. In parallel with the whole thereof is furthermore connected a. large resistance RaY which comprises several taps and from which may be taken voltages of different values which'.

may be supplied to the tone generators.

This circuit-arrangement operates as follows: The charge imparted to the condenser C2 by the above-described switch S1 begins to now away from the condenser through thevarous elements yconnected in parallel with this condenser. The; condenser C3 is completely discharged and consequently acts at first as a short-circuit for theY resistance R3. During the rst discharge period the variation of the voltage at the condenser C2 is consequently only determined by the discharge current whose intensity is determined bytheresistance R2. ri`he condenser C3 is charged by this discharge current and at the end of this charging period the condenser C3 acts as an insulator. The discharge current of the-condenser C1 is consequently determined further onby theresistances R2 and R3 connected in series. Now the, discharge consequently takes placeY less-'rap'- idly than during the nrst discharge period in which only R2 was decisive for the intensity of the discharge current.

Fig. 2 shows the variation of the voltage across the resistance Rs; this curve of voltage was obtained with a circuit arrangement according to Fig. 1. During the period of time t1 in which the condenser C2 is charged the voltage increases rapidly according to curve I3 and attains a maximum value. Immediately afterwards there follows a rrapid discharge which, as has .been described hereinbefore, is practically only determinedby` the resistance R2. Now the voltage I4 decrease during the period of time tz according to an e-function. At the end of the period t2 the condenser C3' is charged so that now the discharge. takes place less rapidly according to curve I5 during a period of time t3. This Voltage also varies according to an e-function with a small exponent since the intensity of the discharge current is less than during the period t2. Theoretically the period ts would have an innite length, the curve I5 asymptotically approaching` the abscissa. The voltage decrease practically to a value below the threshold of the ampliersf and the reproducing device' which are utilizedzsoA that after some time the tone has sounded out.

Jointly the condensers C4 and C5 the resistances R5 and R7 form a so-called click lter. The voltage impulses which may be produced in the circuit at the contact I2 due tothe arm oft the lever S1 being switched on and oil cannot. penetrate, owing to the presence of the filter; as far as to the resistance Ra. These voltage impulses arev consequently not made audible inthe sound reproduction.

The varying voltage developed across resistance- R8 is applied to control the intensity of the tone produced by operating the key K of the instru-y ment. This may be accomplished in a well knownmanner by means of a conventional vacuum tube oscillation generator Il, the voltage developed across resistance Re being impressed as aA variable by keys` and having for each` key thereof a system whose varying output potential controls the intensity of the note produced by striking said: keysaid system comprising a capacitance, means to charge said capacitance to a potential level' proportional to the velocity of the downstroke of said key; and anY impedance connected in parallel-,with said' capacitance for discharging said capacitance, said impedance having iirst and second; values successively in the course of discharge, said first value effecting a decrease in the poten,- tial of said capacitance according Vto an e function having a given exponent, saidi second value eiecting a decrease in potentialV according to. an e function having a relatively small exponent.

2. In an electrical musical instrument playable by keys and having for each key thereof a system whose varying output potential controls the irrtensity of the note produced by striking said key,y said circuit comprising a rst capacitance; means; to charge said irst capacitance to a potential level proportional to the velocity of the downstroke of said key, and an impedance connected in parallel with said rst capacitance, said im.- pedanceA being constituted by a resistance element and a second capacitance shunted-across` yonly a-portionY ofsaid element.

3. In an electrical musical instrument playable by keys and having for each key thereof a system whose varying output potential controls the intensity of the note produced by striking said key, said key being displaced during the downstroke thereof from a rest position to a depressed position and during the release stroke from the depressed position to the rest position, said system comprising a source of constant potential, first and second capacitances, a first discharge impedance, switching means operatively coupled to the key and arranged to connect said first capacitance in the rest position of said key across said source to be charged thereby, and successively in the course of the downstroke across said rst impedance for a period depending on the velocity of said downstroke and then in parallel with said second capacitance, said first capacitance being disconnected by said switching means both in the depressed position and in the course of the release stroke, and a second impedance connected in parallel with said second capacitance, said second impedance being constituted by a resistance element and a third capacitance shunted across only a portion of said element.

4. In an electrical musical instrument playable by keys and having for each key thereof a system Whose varying output potential controls the inten-sity of the note yielded in a tone generator by striking said key, said key being displaced during the downstroke thereof from a rest position to a depressed position and during the release stroke from the depressed position to the rest position, said system comprising a source of constant potential, first and second capacitances, a rst discharge impedance, switching means operatively coupled to the key and arranged to connect said first capacitance in the rest position of said key across said -source to be charged thereby, and successively in the course of the downstroke across said first impedance for a period depending on the velocity of said downstroke and then in parallel with said second capacitance, said irst capacitance being disconnected by said switching means both in the depressed position f and in the course of the release stroke, a second discharge impedance connected in parallel with said second capacitance, said impedance being constituted by a resistance element and a third capacitance shunted across only a portion of said element, and means for applying the varying potential developed across said second impedance to the tone generator to control the amplitude thereof in accordance with said varying potential.

5. In an electrical musical instrument playable by keys and having for each key thereof a system whose varying output potential control the intensity of the note produced by striking said key, v said key being displaced during the downstroke thereof from a rest position to a depressed position and during the release stroke from the depressed position to the rest position, said system comprising a source of constant potential, first and second capacitances, a rst discharge impedance, switching means operatively coupled to the key and arranged to connect said first capacitance in the rest position of said key across said source to be charged thereby, and successively in the course of the downstroke across said first impedance for a period depending on the velocity of the downstroke and then in parallel with said second capacitance, said first capacitance being disconnected by said switching means both in the depressed position and in the course of the release stroke, a second discharge impedance connected in parallel with said second capacitance, said second impedance being constituted by a resistance element and a third capacitance shunte'd across only a portion of Said element, and a shorting switch operatively coupled to sai key and connected across said third capacitance, said switch being closed at the rest position of the key and in the course of the downstroke preceding the connection by said switching means of said rst capacitance in parallel with said second capacitance, said switch being open for the remainder of said downstroke and at the depressed position and in the course of the release stroke.

6. In `an electrical musical instrument playable by keys and having for each key thereof a system Whose varying output potential control the intensity of the note produced by striking said key, said key being displaced during the downstroke thereof from a rest position to a depressed position and during the release stroke from the depressed position to the rest position, said system comprising a source of constant potential, rst and second capacitances, va first discharge impedance, switching means operatively coupled to the key and arranged to connect said first capacitance in the rest position of said key across said source to be charged thereby, and successively in the course of the downstroke across said first impedance for a period depending on the velocity of the downstroke and then in parallel with said second capacitance, said rst capacitance being disconnected by said switching means both in the depressed position and in the course of the release stroke, a second discharge impedance connected across said second capacitance, said second impedance being constituted by a resistance element and a third capacitance shunted across only a portion of said element,

a shorting switch operatively coupled to said key and connected across said third capacitance, said switch being closed at the rest position of the key and in the course of the downstroke preceding the connection by said switching means of said first capacitance in parallel with said second capacitance, said switch being open for the remainder of said downstroke and at the depressed position and in the course of the release stroke, an output potentiometer, and a click lter network arranged to couple said second impedance to said output potentiometer.

FREDERIK KERKHOF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,126,464 Hammond Aug. 9, 1938 2,126,682 Hammond Aug. 9, 1938 2,214,764 Hammond Sept. 17, 1940 2,302,457 Midgley et al Nov. 17, 1942 

